1. Field of the Invention
The invention relates to the field of piezoelectric ceramics. In particular the invention relates to polycrystalline ceramics with high piezoelectric coefficients in thin-layer geometry.
2. Description of the Related Technology
Piezoelectric ceramics such as quartz and lead zirconate titanate (PZT) are the primary component in most actuator applications, which command a multi-billion dollar annual market. PZT dominates the current actuator market because of its high piezoelectric coefficients with d33 ranging from 100 to 700 and d31 ranging from −50 to −300 pm/V. The coefficients d33 and d31 measure the ratio of the strain parallel and perpendicular to the direction of the electric field, respectively. In general, the magnitude of d33 is roughly twice that of d31. For comparison, the −d31 of quartz is less than 10 pm/V.
However, even with such high piezoelectric coefficients, for a typical 1 mm thick plate, the strains generated by 1000 V are still less than 0.1% in PZT's. The market demand for high-strain actuators has fueled intense research interest in developing piezoelectrics with high piezoelectric coefficients (that is, higher than those of commercial PZT)
For polycrystalline piezoelectric ceramics, including PZT, to be useful, they must be polarized in order to have high piezoelectric coefficients. Before polarization, the orientations of domains are random with no net polarization. After polarization, many domains are aligned or switched to the direction of the applied electric field resulting in a finite polarization. However, the domains in polycrystalline materials are not as easily aligned as in a single crystal.
The piezoelectric behavior of a polarized polycrystalline material under an electric field comes from three effects: the intrinsic piezoelectric effect, the domain wall motion, and the electrostrictive effect. The intrinsic piezoelectric effect is related to the deformation of the lattice structure by the applied electric field. The intrinsic piezoelectric effect is generally small. The electrostrictive deformation is proportional to the square of the applied electric field and is also generally small. The main effect produced by the electric field comes from the domain wall motion. When the domain walls move under an electric field, i.e., domain switching, the net polarization of the sample changes thereby resulting in deformation of the material. Only non-180° domain switching causes dimensional changes, whereas 180° domain switching does not. Domain wall motion is known to be influenced by point defects, grain boundaries, microstructures, and compositions.
Due to the demand for increasingly smaller actuators and devices, much effort has been devoted to developing thin-film-based microactuators and microsensors. Most of the piezoelectric thin films investigated were grown on a silicon-based substrate for integration with the silicon circuitry. However, after more than one decade of development, thin films generally exhibited a smaller piezoelectric coefficient than the bulk material due to substrate pinning that seriously hinders domain-wall motion in the film geometry. For example, bulk lead zirconate titanate (PZT) has a d33 of about 500 pm/V, while PZT thin films exhibit a d33 of about 100-200 pm/V.1,2,3 The lower piezoelectric coefficient in thin films is generally attributed to the clamping effect of the substrate.
Recently, a major breakthrough for high-strain piezoelectric ceramics was the development of single crystalline piezoelectric materials. For example, specially cut (001) lead zirconate niobate-lead titanate (PZN-PT) single crystals have a d33 of 2500 pm/V.4 In comparison, PMN-PT bulk ceramics have a d33 about 720 pm/V.5,6 (010)-cut PMN-PT single crystals have a d33 greater than 2000 pm/V and a d31 of −930 pm/V.7 PZN-PT single crystal materials have a d33 on the order of 2000 pm/V, significantly higher than that of its polycrystalline counterpart. This is because the domains in a single crystal can be more easily aligned due to the transformation from a rhombohedral to a tetragonal structure with application of a sufficiently large electric field.
Even though single crystal piezoelectric materials have high piezoelectric coefficients, they are difficult to process. Specialized growth methods have to be designed and the size of the crystals is limited. Furthermore, only a small fraction of piezoelectric materials can be grown into a single crystal. For example, the most popular piezoelectric, PZT, cannot currently be grown into a single crystal. Due to the scarcity of single crystal piezoelectric materials, their price is very high as well. Furthermore, single crystal materials are macroscopic in size. They are difficult to miniaturize for many MEMS (microelectro-mechanical systems) applications.
Therefore, there exists a need for providing polycrystalline ceramics with high piezoelectric coefficients in thin-layer geometry.